The light extraction efficiency (ExE) of LED devices is limited by total internal reflections (TIR) in the high refractive index (n) epitaxial layers, where light is generated. In order to mitigate this TIR, high refractive index encapsulation dome lenses are widely employed as they can effectively boost ExE by as much as 20%. The refractive index of a GaN based LED is between 2-3, and the index of the encapsulation material is typically between 1.4-1.8 to provide a good transition between the GaN and air.
In LED die architectures such Thin-Film-Flip-Chip (TFFC) direct emitters, light is primarily extracted from a single side of the epitaxial layers. The light extraction side is commonly the one exposed to the air after the growth substrate is removed (e.g., by a laser-assisted lift-off process). With a proper surface roughening treatment of the light extraction surface, the extraction efficiency in such direct emitters after dome encapsulation can be as high as 80%, even in cases of rather poor submount reflectivity, since there is little downward side light from the thin epitaxial layers.
The situation can significantly change in Flip-Chip (FC) architectures, such as in Patterned Sapphire Substrate (PSS) based LEDs, where the sapphire growth substrate is not removed but kept for various reasons. In such a device, light from the epitaxial layers enters the transparent substrate, and the opposite surface of the substrate emits the light. Light must thus be extracted from the epitaxial material (e.g., GaN) into the substrate, and from the substrate into either the air or the dome encapsulation material and then air. The field emission of such LED device is much wider, since a great portion of the light is extracted from the four lateral sides of the substrate (typically a couple of hundred microns thick). A good portion of the extracted lateral emission is directed downward at the submount, where it gets reflected back with losses. As such, FC architectures such as PSS based dies rely on high reflective package submounts to offer extraction efficiencies comparable to those of TFFCs. An example of such an FC based LED device is shown in FIGS. 1 and 2, where FIG. 2 is a left side view of the device of FIG. 1.
In FIGS. 1 and 2, LED epitaxial layers 12, including an active layer, are grown on a patterned (roughened) sapphire substrate 14 surface. The epitaxial layers 12 are not identified in FIG. 1 for simplicity and are much thinner relative to the substrate 14 than shown in FIG. 2.
Reflective metal electrodes 15A and 15B are formed on the bottom surface of the LED die 16, and the electrodes are directly bonded to anode and cathode metal pads 18A and 18B on a submount 22. The body of the submount 22 may be ceramic or some other thermally conductive material. The pads 18A and 18B are connected, using vias internal to the submount 22, to associated bottom metal pads 23 (only one pad 23 is shown in the side view). The bottom pads 23 may be later soldered to a printed circuit board. The metal pads 18A and 18B may be reflective to reflect light (such as light ray 26) that is emitted from the lateral sides of the substrate 14. Note how, in FIG. 2, the light ray 26 emitted by the epitaxial layers 12 impinges on the substrate 14 top surface at a shallow angle, is reflected by TIR, exits the side wall of the substrate 14, and is reflected upward by the pad 18A on the submount 22. Hence, the light ray 26 is attenuated by each reflection and the extra distance traveled within the various materials.
A domed lens 28 is then affixed over the LED die 16 either by molding or by affixing a pre-formed lens over the LED die 16 with an adhesive encapsulant (e.g., silicone or epoxy).
It is known that increasing the thickness of the substrate 14, perhaps by as much as 800 microns, yields ExE gains, but that creates fabrication problems and increases cost. With a thicker substrate 14, the light that impinges on its top surface is at an angle closer to the normal angle and thus less TIR occurs. Further, more shallow-angle light is emitted from the thick substrate's sidewalls. This creates a wide emission angle that may not be desirable for some applications.
Thus, what is needed is a technique to increase the light extraction efficiency (ExE) of an LED device without having to increase the thickness of the growth substrate.